Anhydrite has long
been thought to occlude pore space in dolostones producing a tight, nonproductive
carbonate. Although this is true for bedded anhydrite and pore filling
anhydrite, our study of Permian dolostone reservoirs of West Texas demonstrates
that it is not true for poikilotopic anhydrite. Poikilotopic anhydrite
is the most common form of anhydrite in Permian dolostone reservoirs of
West Texas. It occurs in patches, commonly with dolomite inclusions, several
millimeters to centimeters across and both fills interparticle porosity
and replaces the host carbonate. Cross plots of interparticle-porosity
and permeability measurements from medium crystalline mud-dominated dolostones
and grain-dominated dolopackstones that have considerable amounts of poikilotopic
anhydrite tend to plot in the petrophysical class 1 field rather than
in the class 2 field as predicted by Lucia (1995). Also, capillary pressure
data from these samples show anomalously large pore throats for a class
2 fabric.

These observations
suggest that poikilotopic anhydrite reduces porosity but does not reduce
pore-throat size. Permeability is fundamentally controlled by pore-throat
size and remains near constant whereas the pore filling aspect of patchy
anhydrite reduces porosity. This effect causes the data points to shift
to the left, from the class 2 to the class 1 field. This effect has been
verified by initial results from detailed flow simulations that compare
the permeability of a dolostone with and without patches of anhydrite.
Therefore, we suggest that poikilotopic anhydrite, and also anhydrite
nodules, does not form dense, nonproductive dolomite but actually enhances
petrophysical properties.